A large number of materials in a moist state after a production process must be dried. Such materials include salts which contain mechanically and/or chemically bonded moisture.
In most of the drying processes which have been used heretofore, the feed is charged in a dissolved, molten or solid state.
For drying a salt which contains water of crystallization, such as magnesium chloride, the concentrated solution or the melt can be treated with hot gases flowing in a countercurrent. Apart from numerous other examples, the drying of iron sulfateheptahydrate, which is obtained particularly from pickling plants or in the processing of ilmenite and must usually be dried to monohydrate before it is processed further, is of great significance. The iron sulfate solution may be atomized into hot drying gases. This results in a product which contains less than 2 moles or water of crystallization. Instead of a solution, a suspension which is kept at a temperature of 65.degree.-90.degree. C. and which has a composition corresponding approximately to the formula FeSO.sub.4.7-10H.sub.2 0 can be spray-dried in counterflowing drying gases at a temperature of 150.degree.-900.degree. C. Iron sulfateheptahydrate may also be dried in a fluidized-bed reactor, which is operated with the exhaust gases from a decomposition reactor. In another process, the iron sulfateheptahydrate in a comminuted state is exposed to hot gases so that monohydrate is formed within a very short time. The process is controlled to give a product having a particle size below 1600 mesh.
The main disadvantages of these known processes for treating salts which are molten to remove their water of crystallization resides in that the dried product has a very small particle size so that the separation of the product involves a relatively high expenditure. Also, it is difficult to handle, store or process the dried product because it has such a small particle size.